Relaxation oscillators work according to the principle of charging and discharging one or several capacitors to create an oscillation phenomenon. Relaxation oscillators may be used, for example, in phase locked loops to demodulate signals or to recover carriers in telecommunication systems. One disadvantage of relaxation oscillators is that they generally have a relatively high phase noise. For example, for an operating frequency of about 100 MHz, relaxation oscillators have a phase noise of between -80 dBc/Hz and -100 dBc/Hz at 1 MHz from the carrier frequency.
A voltage controlled relaxation oscillator with a high operating frequency (typically 100 MHz) and with a relatively low phase noise (typically -118 dBc/Hz at 1 MHz from the carrier frequency) is described in an article by J. G. Sneep and D. J. M. Werhoeven entitled "Design of a low-noise 100 MHz balanced Schmitt-Trigger oscillator" (ESSCIR 1989, Vienna Austria, pp. 284-287). The block diagram for this type of voltage controlled oscillator is shown in FIG. 1. This oscillator comprises:
a first capacitor C.sub.1 at the terminals of which there is a voltage V.sub.1, PA1 a second capacitor C.sub.2 at the terminals of which there is a voltage V.sub.2, PA1 a charge circuit for capacitor C.sub.1, including a current generator 4 connected to the oscillator power supply voltage +V.sub.dc, PA1 a charge circuit for capacitor C.sub.2 comprising a current generator 5 also connected to the power supply voltage +V.sub.dc, PA1 a current generator 2, used to discharge capacitors C.sub.1 and C.sub.2 alternately, PA1 a switch 1, which alternately either charges capacitor C.sub.1 and discharges capacitor C.sub.2, or charges capacitor C.sub.2 and discharges capacitor C.sub.1, under the action of a control signal K, PA1 relaxation means 3, to generate the switch control signal from voltages V.sub.1 and V.sub.2, and PA1 regulation means 6, 7, such that voltages V.sub.1 and V.sub.2 applied to the relaxation means are approximately equal to a reference voltage V.sub.ref. PA1 a capacitor at the terminals of which there is a voltage V.sub.1, PA1 a circuit that charges the capacitor from a power supply voltage +V.sub.dC, PA1 a circuit that discharges the capacitor, PA1 a switch that alternately charges and discharges the capacitor under the action of a control signal, PA1 relaxation means for generating the switch control signal from voltage V.sub.1, and PA1 regulation means, such that the voltage V.sub.1 applied to the relaxation means is approximately equal to a reference voltage V.sub.ref.
The oscillator frequency is controlled by a control signals CD for respective currents in current generators 4 and 5. Current generators 4 and 5 comprise transistors, of which the parasitic capacitances are sensitive to any variations in the power supply voltage +V.sub.dC. To avoid possible disturbances as a result of the sensitivity, expensive and large filter devices, such as, first order low pass filters requiring the use of large components, then have to be placed between the power supply and current generators.
Furthermore, one possible way of improving oscillator phase noise performance, if required, is to place decay resistances on emitters of the transistors used in the current generators. The effect of decay resistances being present is to increase the potential drop at the terminals of the current generators. The result is a reduction in the voltage range available for operation of the oscillator.
Regulation means comprise a circuit 6 that takes the average of voltages V.sub.1 and V.sub.2 and a comparator 7 that receives the average of voltages V.sub.1 and V.sub.2 on a first input, and a reference voltage V.sub.ref on a second input. The signal output from comparator 7 is applied to control current generator 2.
The oscillation frequency depends on the value of the current output from the current generator 2. It follows that the regulation circuit acts directly on the oscillation frequency of the oscillator. The noise from the various components that make up the regulation loop then directly affects the oscillator phase noise.